This invention relates to a shutter type digital clock. Prior art display devices utilized in the shutter type digital clocks are disclosed, for example, in U.S. Pat. Nos. 3,399,474, 3,683,523, 3,721,087, and 3,965,668.
The display device disclosed in U.S. Pat. No. 3,399,474 comprises four laminated sliding plates each made of an opaque plate, and a plurality of light reflecting layers and slits formed on the surface of the sliding plates. These sliding plates are operated by a suitable operating member to display digits by the light reflecting layers so that it is impossible to construct the display device as an interally illuminated type. Accordingly, the digits are illuminated by the light impinging thereon through openings of a display substrate positioned in front of the sliding plates so that the displayed digits that is the light reflected thereby can been seen in a limited direction and regions. Moreover, the reflected light is vary faint so that it is not only impossible to have brilliant display but also to display when the filament of the illuminating lamp breaks.
To obviate the disadvantages described above display devices as disclosed in U.S. Pat. Nos. 3,683,523 and 3,721,087 have been proposed. The display device shown in these U.S. Patents comprises a display substrate formed with a plurality of display digit openings which are arranged in the form of a letter 8, a shutter plate disposed on the rear side of the substrate in the horizontal direction for opening and closing the display digit openings in the vertical direction and another shutter plate movable in the vertical direction for opening and closing openings in the horizontal direction. Although this display device is advantageous in that it requires only two shutter plates, it is necessary to index the horizontally movably shutter plates to one of 6 positions and to index the vertically movable shutter plate to one of 5 positions. Accordingly, it is necessary to work the cam control mechanism for operating the shutter plates to have extremely high accuracies, and even a small dimensional error causes erroneous display.
The display device disclosed in the applicants U.S. Pat. No. 3,965,668 comprises a display substrate formed with a plurality of display digit openings which are arranged in the form of a letter 8, a first shutter plate positioned to the rear of the display substrate and movable in the vertical direction for opening and closing the display digit openings at the upper and lower sides, a second shutter plate movable in the vertical direction for opening and closing the display digit openings at the middle portion, a third shutter plate movable in the horizontal direction for opening the display digit openings at both upper sides, and a fourth shutter plates movable in the horizontal direction for opening and closing the display digit openings at both lower sides. With this construction, although it is necessary to provide four shutter plates since each shutter plate is required to be indexed to one of at most three positions, there is an advantage of simplifying the construction of the cam control mechanism. However, in any one of the prior art display devices described above where the number of the shutter plates is decreased, the shutter plates overlap with each other on the rear side of the display substrate so that not only the thickness of the display member increases but also the configuration of the digits displayed would be deformed during the coarse of changing the digits. Moreover, in the display device of the type described above wherein a lamp is contained in the display device and leak of light through the digit opening is controlled by the shutter plates, even when respective shutter plates are not positioned on a plane there is no trouble, but when the filament of the lamp breaks or where the ground color of the back plate is used for display, the construction is not advantageous.